1. Field of the Invention
This invention relates to additives for the proton conducting polymer electrolyte used in catalyst layers in fuel cells. In particular, it relates to additives for reducing catalyst dissolution, especially from the cathode.
2. Description of the Related Art
Proton exchange membrane fuel cells (PEMFCs) convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). MEA durability is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive applications, an MEA is required to demonstrate durability of about 6,000 hours.
Degradation of the catalyst during operation of the fuel cell, especially during start up or shut down in which the transient potential at the cathode could be over 1 V, is a critical issue to address in order to maintain cell performance without significant decay for over 5,000-6,000 hours of operation. Cathode catalyst layer degradation and the associated decay in fuel cell performance may be caused by 1) loss of Pt catalyst surface area due to formation of larger particles from smaller particle Pt dissolution (Ostwald Ripening) or coalescence of Pt nanoparticles by thermal motion (sintering) or loss of the carbon support typically employed due to carbon corrosion; 2) Pt dissolution and migration into the catalyst electrolyte, membranes, or washed out altogether in by-product water; 3) contamination of ionomer electrolyte by dissolved metal ions from catalyst; and 4) contamination of the catalyst surface by chemicals resulting from ionomer degradation.
One of ways to mitigation the cathode catalyst degradation is to decrease the dissolution of Pt catalyst during fuel cell operation. Ptz+ ions most probably do not exist as free species in the ionomer phase or in solution. They have to be associated with counter ions in order to maintain charge neutrality. Fluorine (F−), chlorine (Cl−) and other halogen ions are common counter ions that can form complexes with Ptz+ ions to make them water soluble. Halide anions are strong ligands and they may promote ligand exchange with O2− and de-passivate a Pt oxide layer, thereby accelerating the dissolution of the Pt. Halide anions can come from residual Pt chloride that is commonly used in the production of Pt nanoparticles. F− can be released from the perfluorosulfonic acid (PFSA) polymer membrane or ionomer due to the ionomer degradation in the catalyst layer during fuel cell operation. A decrease of halide anions in the catalyst layer could lower the Pt dissolution, thereby mitigating cathode catalyst layer degradation. P. Trogadas and V. Ramani added a peroxide decomposition catalyst (MnO2) into the anode and cathode electrocatalysts to facilitate both electrochemical oxygen reduction and hydrogen peroxide decomposition to water and oxygen [P. Trogadas, V. Ramani, Journal of Power Sources 174 (2007) 159-163]. By lowering hydrogen peroxide concentration within the electrode, the fluorine release rate associated with the decomposition was decreased. However, the drawback was a loss of catalyst activity by adding the MnO2 in the catalyst layer.
In published patent application WO 2008/032802 A1, it was claimed that a complex of Pt having ligands such as acetylacetone and ethylene di-amine tetra-acetic acid (EDTA) as coordinating atoms could mitigate Pt dissolution from catalyst surface due to equilibrium shifting toward solid Pt. In the disclosed voltage cycling test (from 0.05-1.2 V in nitrogen and hydrogen), 10% of acetylacetone in the cathode catalyst layer showed the lowest voltage decay. However, such highly water soluble ligands (acetylacetone solubility: 160 g in one liter of water) are not expected to stay in the catalyst layer long during operation. They will be washed out soon after the load is applied to the fuel cell. Fluorine ions and fluoride fragments, for example, pentafluoropropionic acid (PFPA, CF3CF2COOH), may form during fuel cell operation as a result of membrane degradation. Fluorine ions formed in the membrane may diffuse into the catalyst layer and accelerate Pt dissolution. Fluoride fragments may contaminate the Pt surface and lower its activity. Therefore, decreasing the fluorine release rate associated with degradation of the membrane by increasing its chemical stability could mitigate catalyst layer degradation.
Different additives to the membrane electrolyte have been studied for purposes of decreasing the fluorine release rate. These additives include: 1) metal elements or compositions containing metal elements or metal alloys that act as a free radical scavenger or a hydrogen peroxide decomposition catalyst (e.g. US2004043283); 2) phenol or phenol derivatives that can be a small molecule or a polymer (e.g. US2006046120); 3) organic crown compounds (e.g. US20060222921) or macrocyclic compounds containing metal or metalloids (e.g. WO2007144633); and 4) cation chelating agents to reduce formation of free radicals (e.g. U.S. Pat. No. 6,607,856).
Additives are also disclosed in WO2005060039 to address the problem in PEM fuel cell durability of premature failure of the ion-exchange membrane. The degradation of the ion-exchange membrane by reactive hydrogen peroxide species can be reduced or eliminated by the presence of an additive in the anode, cathode or ion-exchange membrane. The additive may be a radical scavenger, a membrane cross-linker, a hydrogen peroxide decomposition catalyst and/or a hydrogen peroxide stabilizer. The presence of the additive in the membrane electrode assembly (MEA) may however result in reduced performance of the PEM fuel cell. In particular, suggested additives include an organometallic Mn (II) or Mn (III) complex having an organic ligand selected from CyDTA, ENTMP, gluconate, N,N′-bis(salicylidene)propylenediamine, porphoryns, phthalocyanines, phenanthroline, hydrazine, pyrocatechol-3,5-disulphonic acid disodium salt, triethylenetetraamine, shiff base macrocycles and EDDA.
In commonly owned U.S. patent application Ser. No. 12/615,671, with the title “Composite Proton Conducting Membrane with Low Degradation and Membrane Electrode Assembly for Fuel Cells” and filed on Nov. 10, 2009, certain ligand additives (e.g. 1,10-phenanthroline or 2,2′-bipyridine) were disclosed that meet many of these needs. The use of these ligand additives in the membrane and/or catalyst layers can improve durability but, depending on testing conditions, there may be a modest penalty in fuel cell performance (e.g. 3 times better stability might be obtained but with a 20 mV loss in voltage under load). Preferably, both durability and performance of fuel cells would be improved with appropriate additives.
In PCT patent application PCT/EP2010/006836, also commonly owned by the present applicant, certain additives were disclosed for polymer electrolytes in order to improve both durability and performance. The additives were chemical complexes comprising certain metal and organic ligand components.
There remains a continuing need for improved additives to reduce MEA degradation and, in particular, to prevent dissolution of electrode catalysts during operation. This invention fulfills these needs and provides further related advantages.